This invention relates to neutron-neutron well logging and more particularly to an improved method of and system for determining the porosity of subsurface formations surrounding a borehole.
In such neutron-neutron logging, a fast neutron source irradiates the formations surrounding the borehole. The resulting secondary radiation is preferably measured by thermal neutron detectors axially spaced from such source within the borehole. The basic principles of such neutron-neutron logging have been described in an article entitled "Dual-Spaced Neutron Logging For Porosity" by L. S. Allen, C. W. Tittle, W. R. Mills, and R. L. Caldwell in GEOPHYSICS, Vol. 32, No. 1, pp. 60-68 (February, 1967). Briefly, a two-group neutron diffusion theory describes such resulting secondary radiation, these two groups being epithermal neutrons and thermal neutrons.
For a point neutron source in an infinite, homogeneous medium, this secondary radiation can be represented as follows: ##EQU1## WHERE, .PHI..sub.T IS THE THERMAL NEUTRON FLUX,
R is the radial distance measured from the source, PA1 Q is the point neutron source strength, PA1 D is the thermal neutron diffusion coefficient, PA1 L.sub.e is the epithermal neutron parameter (slowing down length), and PA1 L.sub.t is the thermal neutron parameter (diffusion length).
The epithermal neutron parameter L.sub.e of the formation principally is determined by the concentration of hydrogen in the formation, and hydrogen content is related to the porosity of the formation. The thermal neutron parameter L.sub.t of the formation also is related to porosity but is strongly affected by the total macroscopic absorption cross section of the formation, that is, the macroscopic absorption cross sections of both the rock matrix and fluids within the formation. Firstly, the macroscopic absorption cross section of the formation fluid is affected by the salinity of the fluid and is significantly reduced when the pore spaces of the formation contain salt water rather than oil. The chlorine present in the salt water has a large macroscopic absorption cross section for thermal neutrons and, consequently, reduces the number of thermal neutrons returning to the borehole as secondary radiation. At the same time, the absorption of thermal neutrons by the chlorine effects an increase in the number of thermal neutron capture gamma rays returning to the borehole as secondary radiation. Secondly, the macroscopic absorption cross section of the formation rock matrix is affected by certain trace elements common in shales and other sedimentary rocks. For example, boron and gadolinium absorb thermal neutrons strongly to reduce the number of thermal neutrons returning to the borehole as secondary radiation. Consequently, secondary radiation resulting from irradiation of the formation surrounding a borehole by means of a fast neutron source is affected by both thermal and epithermal neutron parameters and is an indication of both the porosity and the macroscopic absorption cross section of the formation.
In the process of measuring such secondary radiation by thermal neutron detectors within the borehole, certain conditions within the borehole itself adversely affect the thermal neutron flux measurements made by thermal neutron detectors. These include borehole size, type of fluid in the borehole, and eccentricity of location of the logging tool in the borehole. By taking the ratio of the thermal neutron fluxes detected by two spaced-apart thermal neutron detectors, a measurement is made of the secondary radiation from the formation that is only slightly affected by such adverse borehole effects. Consequently, such a ratio of the thermal neutron fluxes detected at two such spacedapart positions within the borehole is predominantly indicative of the epithermal neutron L.sub.e and thermal neutron L.sub.t parameters of the formation, that is, the porosity and macroscopic absorption cross section of the formation.
This ratio of the thermal neutron fluxes is to a greater extent dependent upon the epithermal neutron parameter than the thermal neutron parameter. As a result, such a ratio has been utilized in the past as an apparent indication of the porosity of the formation having been logged. However, to utilize such a ratio of the outputs of two thermal neutron detectors as a correct indication of formation porosity, the varying effects of the total formation macroscopic absorption cross section must be compensated for. In U.S. Pat. No. 3,491,238 to L. S. Allen there is described a technique and system for determining a correct porosity measurement by essentially eliminating the macroscopic absorption cross-section effect on the thermal neutron flux measurements of each two spaced-apart thermal neutron detectors. As noted in such patent, at large source-to-detector spacings, the thermal neutron flux is governed by the epithermal neutron parameter because this parameter is larger than the thermal neutron parameter. Therefore, by locating two thermal neutron detectors at realtively large distances from the neutron source, and measuring the ratio of the detector outputs, the thermal neutron parameter effect is eliminated from the thermal neutron flux ratio measured by these detectors. The ratio of the outputs from the two detectors is therefore unaffected by the formation macroscopic absorption cross section and is a correct indication of formation porosity.
Many conventional borehole logging systems, however, employ spaced-apart thermal neutron detectors that are not located sufficiently far from the neutron source so as to eliminate the effects of variations in the thermal neutron flux measurements of the thermal neutron detectors. Consequently, the ratio of the outputs of the thermal neutron detectors is only an apparent indication of the porosity of the formation. It is, therefore, an object of the present invention to provide for a new and improved technique and system by which the formation macroscopic absorption cross-section effect on the thermal neutron flux measurements can be compensated for to provide for a porosity measurement that is a correct indication of the porosity of the formation.